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I was just reading about the working of the DME the other day and then I read a para which said:

The aircraft interrogator "locks on" to the DME ground station once it recognizes a particular pulse sequence having the same spacing as the original interrogation sequence

Can anybody please elaborate on the above statement? I just want to know what is "lock on" and how does an aircraft interrogator lock on to a DME ground station?

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"Lock-on" just refers to a unit receiving multiple different response signals in response to its interrogation broadcasts, scanning for the signal it's looking for, and when it finds the signal who's encoding matches its channel selection, it starts to process that signal and ignores all others. It's locked on to that signal.

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The ground station transmits responses to interrogations from other aircraft and randomly timed signals that look just like responses, so an aircraft has to pick responses to its transmissions out of all the other signals that look like they could be. The way it's done is by looking at the ground station transmissions for a while trying to find a time after its interrogations where there's usually a response. (Not all interrogations get responses.) Once the time slot is found the aircraft can calculate the distance to the ground station and only has to track the changes in the distance from then on. It reduces its interrogation rate in accordance with its lighter workload and is locked on.

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